This invention relates to a process for making the alloy, mercury cadmium telluride (Hg.sub.1-x Cd.sub.x Te) wherein x is the mole fraction of CdTe.
Mercury cadmium telluride is presently utilized as an infrared detector and can be utilized in solar cells, integrated electronic and optical circuits, as well as various component devices such as diodes, transistors, modulators, waveguides and lasers. Mercury cadmium telluride is a mixed crystal alloy system of mercury telluride and cadmium telluride which are mutually soluble in all compositions. Since cadmium telluride is a semiconductor and mercury telluride is a semi-metal, the alloy system ranges from semi-metal to semiconductor with an energy gap variation dependent upon the mole fration of cadmium telluride. It is desirable to obtain single crystal and high purity mercury cadmium telluride material from which to fabricate intrinsic infrared detectors or the like. However, the alloys of this system are difficult to synthesize in a controlled process since the alloys of the system are decomposing solids. When the solid phase of the alloy is heated in an open vessel, the components are lost at a rate so that congruent melting is impossible. Thus, prior art processes have prepared these solids in sealed containers generally at about 50 to 100 atmospheres at the liquidus temperature which increases the possibility of explosion.
The liquidus-solidus temperatures of the mercury telluride and cadmium telluride alloys are different, which causes their segregation as the alloy is frozen or solidified from a melt. The resultant point to point variation of the mole fraction of each of these two alloys in the solidified material results in a corresponding variation in energy gap and consequent variation in the electrical and optical properties throughout the material. Also, the alloys are defect solids in which deviations from stoichiometry which may occur from point to point within the material act as electrically active donors and acceptors.
U.S Pat. No. 3,725,135 to Hager et al discloses a process for forming mercury cadmium telluride by enclosing cadmium telluride substrates and mercury telluride in a closed evacuated container which are then heated to produce an atmosphere of excess mercury vapor pressure. This process requires a minimum of about 3 days to produce the desired alloy and, due to the seal-off procedure utilized, increases the probability of contamination. U.S. Pat. No. 3,723,190 to Kruse et al also utilizes a sealed evacuator container. In addition, like the Hagger et al process, the products produced are too thick for electronic devices so that the substrate and part of the alloy layer must be ground, polished or etched to be utilized in an electronic or optical device. Accordingly, it would be highly desirable to provide a process for producing mercury cadmium telluride alloy that permits obtaining a pure product, does not require high pressures and which require only relatively short time periods to produce the alloy.